(1) Field of the Invention
The present invention relates to an impact printer.
Impact printers are divided roughly into two types, one type uses a hammer and strikes printing type to print letters and characters, and the other type is a wire-dot printer which has a plurality of wire-dot pins which print characters composed of image dots printed by the wire-dot pins. Every impact printer includes at least one coil to drive the printing hammer or the wire-dot pins. In this specification, the coil used to drive the printing hammer or the wire-dot pins is called a printing coil. A drive transistor is provided for each printing coil to control a current flow through the coil. A character or a dot is printed when the current flows through the coil.
(2) Description of the Related Art
In an impact printer, when the coil or the transistor become defective due to either an open circuit or a short circuit, a printing operation cannot be carried out. Further, when the current through a short circuit in the transistor continues to flow, the coil driven by the transistor may be damaged by overheating. In order to avoid these problems, a prior art impact printer includes a detection circuit to detect an open circuit or a short circuit in the printing coil or the driving transistor.
For example, Japanese Unexamined Patent Publication (Kokai) No. 51-78932 discloses a detection method for detecting whether or not a hammer driven by a magnetic circuit is operating properly in response to a drive signal. Japanese Unexamined Patent Publication (Kokai) No. 52-85421 discloses a detection apparatus for selectively detecting erroneous operation of a plurality of switching circuits for printing hammer magnets. Japanese Unexamined Patent Publication (Kokai) No. 52-85422 discloses a detection method for detecting erroneous operation of printing hammer magnets. Japanese Unexamined Utility Model Publication (Kokai) No. 54-84026 discloses a pulse motor which includes a detection means for detecting erroneous operation of a switching circuit for a coil. Japanese Unexamined Patent Publication (Kokai) No. 57-108974 discloses a printing apparatus in which short circuits in a plurality of printing magnet coils are selectively detected. Further, U.S. Pat. No. 4,819,556 discloses a fundamental constitution of a line printer having a shuttle on which a plurality of printing elements are arranged.
According to these documents, the reliability of an impact printer is improved. However, the above documents disclose impact printers in which only open circuits and short circuits in a printing coil or a driving transistor can be detected. Namely, these impact printers can detect defects only when printing coils or drive transistors are completely open or short-circuited. However, in practice, defects other than open circuits or short circuits of the printing coils or drive transistors can occur. For example, when a printing coil is partially short-circuited or the distance between a printing coil and the iron core of a magnetic circuit is changed, the driving ability of the printing element decreases. This defect can cause a reduction in print density. In the prior art impact printer, this defect cannot be detected because the printing coil is not completely open or short-circuited.